module Agari where {

  import List;

  type M a = [a];
  
  unitM :: a -> M a;
  unitM a = [a];
  
  bindM :: M a -> (a -> M b) -> M b;
  m `bindM` k = case m of { 
		  []   -> [];
		  x:xs -> k x ++ (xs `bindM` k)
		};
  
  failM :: M a;
  failM = [];

  pick :: Eq a => [a] -> M a;
  pick xs = nub xs;  
  
  remove :: Eq a => a -> [a] -> M [a];
  remove x xs = if x `elem` xs then unitM (delete x xs) else failM;
  
  shuntsu :: [Int] -> M ([Int], [Int]);
  shuntsu xs = pick xs `bindM`    \ y0 ->
	       remove  y0    xs  `bindM` \ ys0 ->
	       remove (y0+1) ys0 `bindM` \ ys1 ->
	       remove (y0+2) ys1 `bindM` \ ys2 ->
	       unitM ([y0, y0+1, y0+2], ys2);

}